Williamson, Matthew J.*; Dauer, Lawrence T.†
Patient release criteria after administration of radioactive materials are based on an estimated dose to a member of the public. The International Commission on Radiation Protection (ICRP) recommends that the public dose limit of 1 mSv per year should be applied to infants, children, and casual visitors and furthers a dose constraint of 5 mSv per episode for relatives, visitors, and caregivers (ICRP 2004). The U.S. regulation on patient release originates with the U.S. Nuclear Regulatory Commission (NRC) in Title 10 of the Code of Federal Regulations Part 35, section 75 (10 CFR 35.75), or similar reference in agreement states. 10 CFR 35.75(a) authorizes a licensee to release from its control any individual who has been administered byproduct material if the total effective dose equivalent to any other individual from exposure to the released individual is not likely to exceed 5 mSv. It requires a licensee to provide the released individual, or the individual’s parent or guardian, with instructions, including written instructions, on actions recommended to maintain doses to other individuals as low as reasonably achievable if the total effective dose equivalent to any other individual is likely to exceed 1 mSv (USNRC 2013). In the U.S., regulatory guidance is issued to facilitate compliance with radiation protection regulations. The current U.S. guidance document for medical licensees is NUREG 1556 Volume 9, Revision 2. Appendix U of this guidance document contains a model procedure for release of patients administered radioactive materials (USNRC 2007). The model for patient release is based on an estimated dose to a member of the public using the gamma constant of the nuclide, an occupancy factor based on half-life, and a distance from the patient of 1 m. This model does not account for biological distribution or removal of the pharmaceutical, self-attenuation from the patient, or any other parameters that might be specific to the patient or the scenario. Situational specific information may be used to better estimate the dose to a member of the public and is recommended as a more accurate estimate of the potential dose to a member of the public (Culver and Dworkin 1992; Al-Haj et al. 2007). Zanzonico et al. (2000) provide a mechanism for specific scenarios of public interaction with patients released from licensee control, which is further reviewed by the NCRP (Zanzonico et al. 2000; NCRP 2007). Licensees may offer a mechanism of compliance with patient release and instruction regulations by simply committing to the model procedures contained in the NUREG. Some suggest that the assumptions in the regulatory guidance are antiquated and therefore recommend an extra-conservative approach to patient release (North et al. 2001; Parthasarathy and Crawford 2002). While others recognize that the current guidance is conservative in its assumptions (DeSantis and Chabot 2001; Rutar et al. 2001; Siegel et al. 2007; Willegaignon et al. 2007; Stabin et al. 2009; de Carvalho et al. 2011), licensees often employ its fundamentals when developing procedures for patient release. The American Thyroid Association recommends periodic reviews of programs and protocols regarding patient release and radiation safety instruction while focusing on sound medical practice and adherence to regulations (Sisson et al. 2011).
The use of unconventional or novel radionuclides for positron emission tomography (PET) is becoming more prevalent in both nuclear medicine diagnosis and therapy (Holland et al. 2010). The basis for patient release and radiation safety instruction is rooted in an estimated dose to a member of the public. Assumptions in regulatory guidance are used often to demonstrate compliance with patient release requirements. The guidance for patient release and radiation safety instruction is reviewed and extended toward novel PET radionuclides.
Siegel traces the history of the 30-mCi patient release rule to the 1950s (Siegel 2000). Recommendations from the Atomic Energy Commission (AEC) included hospitalization of patients with greater than 30 mCi of any nuclide. Siegel notes that hardened regulation was limited to recommendations in licensing guidance and radioactive material license conditions. A review of historical licenses at this institution dating to 1963 confirms such license conditions: “Patients receiving more than 30 millicuries of unsealed gamma sources shall remain hospitalized until the activity is less than 30 millicuries.”
In 1970, the National Council on Radiation Protection (NCRP) Report Number 37 recommended measures and procedures for minimizing unnecessary irradiation with regard for individuals caring for or associating with patients who have received therapeutic doses of radioactive materials (or millicurie range) for diagnostic purposes. The report states the maximum permissible dose to those not exposed occupationally as 500 mrem per year (NCRP 1970). In order to estimate exposure for a given radionuclide over a period of time, eqn (1) was presented:
Eqn (1) employs the gamma constant (Γ) for a particular radionuclide in units of R cm2 (mCi h)−1, administered activity (Q0) in millicuries (mCi), exposure time (t) in days, the physical half-life (T) in days, distance (r) from the point of interest in centimeters, and the conversion factor of 24 h d−1 multiplied by the total integration of decay (1.44). The NCRP states, “In accordance with the degree of accuracy acceptable in these situations, and to facilitate the interpolation or extrapolation of the specifications in this report, the simplifications noted above have been deliberately employed” (NCRP 1970). It is recognized that this model is simplistic and contains many uncertainties. For example, NCRP Report No. 37 does not include occupancy factors, but it does recognize that exposures may not occur for the entire 24 h in a day. The NCRP included activity at discharge and exposure rates at 1 m from the patient for several radionuclides as an estimate for dose to a member of the public. It is recommended that exposure rate rather than activity be used as the mechanism for estimating dose to the public. This report was progressive as compared to previous recommendations on patient release, which were limited to an estimated residual amount of radioactivity in the patient regardless of the nuclide.
The NRC published for comment the rulemaking for patient release in the Federal Register in 1985 (USNRC 1985). Patient release was proposed initially as only an exposure rate criterion of 6 mR h−1 at 1 m from a patient administered a radiopharmaceutical or permanent implant, as discovered in an earlier notice of proposed rulemaking issued by the NRC and the Federal Register notice. Using eqn (1) [the gamma constant in NCRP 37, time to infinity, and solving for D(t)], the estimated dose at 1 m from a patient is 1.67 R. The NRC further assumed an occupancy factor of 8 h d−1 or one-third. Multiplying 1.67 R by one-third yields an estimated exposure of 0.556 R, which is greater than 0.5 R. It is probable that the exposure rate release limit ultimately expressed in the final rule was reduced to 5 mR h−1 at 1 m from the patient due to its resultant estimated exposure of 0.464 mR (i.e., < 0.5R), using the same methodology found in NCRP 37. When the final rule was published in the Federal Register, the 30 mCi residual activity limit was incorporated based on current licensing practice and the recommendations of the NCRP. However, using the NCRP 37 guidance, 30 mCi of 131I estimates an exposure of 0.612 R, in excess of 0.5 R. NCRP 37 suggested a residual activity for 131I of 8 mCi, resulting in an exposure of 0.5 R for an occupancy factor of 1. An occupancy factor of one-third increases the residual activity remaining in the patient to 24 mCi, less than the 30 mCi proposed by the NRC.
The NRC codified release of patients administered radiopharmaceuticals in 10 CFR 35.75 with the release limits of 5 mR h−1 at 1 m from the patient or when the activity remaining in the patient is less than 30 mCi, regardless of the administered radionuclide in 1987 (USNRC 1987) and was echoed in the patient release recommendations of NRC Regulatory Guide (Reg. Guide) 10.8 Revision 2 (USNRC 1987; Siegel 2000). Although the NCRP provided recommendations for patient release related to various radionuclides in their report issued in 1970, the NRC continued with a single activity threshold regardless of the radionuclide.
Carol Marcus initiated a change in thinking while petitioning the NRC regarding the member-of-the-public dose limit (1 mSv) offered in the revised 10 CFR 20 and its application to the release of patients, ultimately leading to the idea of a dose limit for patient release rather than the activity and dose rate limit (USNRC 1990). In 1997, the NRC changed the patient release rule (USNRC 1997a) and issued NUREG 1492 (USNRC 1997b) and Reg. Guide 8.39 (USNRC 1997c), offering mechanisms to comply with the dose-based release criterion of 5 mSv to a member of the public. An additional dose-based criterion of 1 mSv was provided as a threshold for providing patients with radiation safety instruction.
Eqns (2) and (3) (from NUREG 1492 and Reg Guide 8.39) are used to estimate a dose to infinity based on the gamma constant (Γ) and administered activity (Q0). These equations contain slight modifications from the equation in NCRP Report No. 37. Eqn (2) is used for radionuclides with a physical half-life greater than 1 d and assumes an occupancy factor of 0.25. Eqn (3) is used for radionuclides with a physical half-life greater than 1 d and assumes an occupancy factor of 1:
NUREG 1492 and Reg. Guide 8.39 used eqns (2) and (3) for radionuclides encountered routinely in nuclear medicine procedures to develop activity and dose rate release criteria (5 mSv) and thresholds for patient radiation safety instruction (1 mSv). These are guidance for the licensee in determining (1) when the licensee may authorize the release of a patient who has been administered radiopharmaceutical or permanent implants containing radioactive material and (2) when instructions to the patient are required by 10 CFR 35.75(b).
Eqns (2) and (3) may be rewritten, solving for activity (Q0) and where D(∞) is set to equal the dose-based thresholds of 10 CFR 35.75, for radiation safety instruction to a patient or patient release; i.e., 1 mSv and 5 mSv, as demonstrated in eqn (4), respectively. Occupancy is represented by T and is based on the half-life. Occupancy (T) is either 1 or 0.25, as noted in eqns (2) and (3):
International recommendations on patient release stem from the International Atomic Energy Agency (IAEA) and the ICRP. Other organizations are progressive in their recommendations. The European Atomic Energy Community (EURATOM) is an international organization under the European Commission that offers publications and standards on radiation protection. The European Commission published a set of radiation safety standards in 1996 to include public dose limits. They specifically excluded individuals knowingly and willingly helping in the support and comfort of patients undergoing medical diagnosis or treatment from the 1 mSv public annual dose limit (EUR 1996). The European Commission document permits exceeding the 1 mSv public limit for 1 y, provided the average effective dose over five consecutive years does not exceed 1 mSv y−1.
The European Commission later published radiation protection guidance following 131I therapy. The recommendations mimic those published prior for members of the public and continue to exclude persons knowingly and willingly caring for patients undergoing medical diagnosis or treatment from the public dose limit (EUR 1998). They refer to this population as family and close friends. It is recommended that persons not caring for the patient be considered third persons (members of the public), and a 0.3 mSv upper dose limit is recommended. The 1 mSv limit is interpreted as a cumulative limit, and it is suggested that the third person may encounter other manmade exposures during the year. This European Commission document also presents dose constraints for members of the public for different age groups. For the developing fetus and all children, a dose constraint of 1 mSv is presented. The dose constraint for adults up to about 60-y old is 3 mSv. For those over 60-y old, the dose constraint is 15 mSv (EUR 1997).
IAEA Safety Report Series, Report No. 40, Applying Radiation Safety Standards in Nuclear Medicine, is in line with the European Commission public limits; i.e., a public dose limit of 1 mSv y−1 (IAEA 2005). Report No. 40 also permits public exposures above the limit; up to 5 mSv y−1 for special circumstances provided the average dose over five consecutive years does not exceed 1 mSv y−1. This report does not address caregivers and family members. IAEA Report No. 63, Release of Patients after Radionuclide Therapy, restates the public dose limits and permission to exceed the limits and the average not to exceed 1 mSv over 5 y, but it does not address specifically the 5 mSv maximum for the year (IAEA 2009). The report also states that there are no comforter and carer dose limits. The IAEA offers dose constraints for the comforter and carer population adjusted for age; 1 mSv per episode for pregnant women and children up to 10-y-old, 3 mSv per episode for children older than10 y to adults up to 60 y, and 15 mSv for adults older than 60 y. Report No. 63 also presents the dose constraint of 0.3 mSv per episode to third parties.
In 2005, NRC guidance for medical licensees and patient release was consolidated into NUREG 1556 Vol. 9, revised in 2008. For this study, prior to calculating release criteria and thresholds for radiation safety instruction for novel PET radionuclides, the method offered in NUREG 1556 Vol. 9 Rev. 2 Appendix U was verified. The methods include: the estimation of the linear energy absorption coefficient in air (μen); the comparison of published μen data with estimated data; the estimation of exposure rate constants (previously referred to as gamma constant) based on μen, radionuclide decay schemes, and additional assumptions; the comparison of these exposure rate constants with published literature; and the calculation of release criteria and thresholds for radiation safety instruction for novel PET radionuclides.
The radionuclides presented in Appendix U were introduced in NCRP Report No. 37 and further evaluated in NUREG 1492. When not referenced from other sources, exposure rate constants listed in NUREG 1492 were derived empirically based on the photon energy, the photon abundance, and the linear energy absorption coefficient in air (μen) for the energy of interest (USNRC 1997b). Many resources derive and list exposure rate constants, and they vary. In the interest of this model verification, the method presented in current regulatory guidance and basis documents was used to determine the exposure rate constant.
The μen data from NUREG 1492 were estimated from the Radiological Health Handbook (USDHEW 1970) as referenced in NUREG 1492, which is based on data compiled in the Engineering Compendium on Radiation Shielding (Jaeger 1968). The μen is frequently tabulated with large intervals between energies. These large gaps in the tabulated data, combined with a non-linear function with variation in slope, can lead to differences in reported exposure rate constants. In order to better interpret the function, the μen graph was scanned from the Radiological Health Handbook (RHH) (USDHEW 1970) into a portable document format (PDF) and printed on a 3 feet X 4 feet sheet of paper. The generated μen are compared with available data.
NUREG 1492 presented several radionuclides, their photon energies, and energy abundances used in calculating the exposure rate constant. This work recalculates the exposure rate constants in NUREG 1497 using the derived μen and the photon energy and abundances given in the NUREG. A VLOOKUP formula was used in Microsoft® Excel 2007 to facilitate the correlation of photon energy and linear energy absorption coefficients in air. It is noted that NUREG 1492 used a photon energy threshold of 11.3 keV as employed in NCRP Report No. 41. The basis for the 11.3 keV photon energy threshold is referenced as private communication (NCRP 1974).
The exposure rate constants were recalculated again for the radionuclides in NUREG 1492, using the photon energy and abundance from ICRP 107 decay tables (ICRP 2009) that used the derived μen. As in NUREG 1492, photons with energies less than 11.3 keV were not included in the exposure rate calculation.
Smith and Stabin recently published exposure rate constants, as this current work was in progress (Smith and Stabin 2012). They tabulate exposure rate constants for over 1,000 radionuclides based on ICRP 107 decay schemes. A photon energy threshold of 15 keV and an abundance threshold of 0.0001 were used in their calculation of the exposure rate constant. Using the Smith and Stabin energy and abundance threshold values, exposure rate constants were estimated in this work for the radionuclides in NUREG 1492 based on the derived μen in air and ICRP 107 decay tables. The calculated exposure rate constants are compared in NUREG 1492, NUREG 1556 Vol. 9 Rev. 2, and Smith and Stabin (2012).
Exposure rate constants are calculated for a listing of novel radionuclides presented by Holland et al. (2010) using ICRP 107 decay schemes, the derived μen, and energy and abundance thresholds of 15 keV and 0.0001, respectively. These are compared to the exposure rate constants presented by Smith and Stabin (2012). The estimated novel radionuclide exposure rate constants from this work, the half-lives listed in ICRP 107, and the occupancy factor based on the half-life were used to estimate patient release criteria and patient radiation safety instruction thresholds using eqn (4), the ultimate result of this work.
Table 1 presents the μen in air based on a direct visual interpretation of the log-log graph of μen found on page 135 of the Radiological Health Handbook (USDHEW 1970), converted to PDF and plotted on 3 feet X 4 feet paper. The error reported for these data is the reading error. The reading error is estimated as plus or minus half the lowest division on that section of the graph (Bakke 2009). Results are reported to the first figure in the error. Fig. 1 was generated using Microsoft Excel (Microsoft® Office Excel® 2007) and the data from Table 1.
Table 2 compares the estimated μen in air from Table 1 to published values (ICRU 1970; Shleien et al. 1998; NBS Handbook 1964). The ratio of the estimated μen in air from Table 1 to the published work is offered. Data columns are labeled, and the column labels are used to describe the ratio of this work against the published values.
Fig. 2 shows the exposure rate constants for the nine radionuclides that were presented with decay schemes and linear energy-absorption coefficients in NUREG 1492. The figure compares the various exposure rate constants: (1) NUREG 1492; (2) NUREG 1492, μen from Table 1, and the 11.3 keV photon threshold; (3) ICRP 107 decay schemes, μen from Table 1, and the 11.3 keV photon threshold; (4) Smith and Stabin exposure rate constants; and (5) ICRP 107 decay schemes, μen from Table 1, and energy and abundance thresholds of 15 keV and 0.0001, respectively.
Table 3 presents published exposure rate constants from NUREG 1492, NUREG 1556 Vol. 9 Rev. 2., Smith and Stabin (2012), and this work using the ICRP 107 decay schemes; μen from Table 1; and energy and abundance thresholds of 15 keV and 0.0001, respectively. The calculated exposure rate constants are compared to the NUREG 1492, NUREG 1556 Vol. 9 Rev. 2, and Smith and Stabin (2012) values as ratios. Exposure rate constants are maintained in the units (i.e., R mCi h−1 at 1 cm), since they were initially presented in that format.
Exposure rate constants are calculated for a listing of novel radionuclides presented by Holland et al. (2010) using ICRP 107 decay schemes, the derived μen, and energy and abundance thresholds of 15 keV and 0.0001, respectively. These data are presented in Table 4. They are compared to the exposure rate constants by Smith and Stabin (2012). Data are maintained in U.S. units (R mCi h−1 at 1 cm) as presented by Smith and Stabin and previous regulatory guidance documents.
The estimated novel radionuclide exposure rate constants from this work, the half-lives listed in ICRP 107, and the occupancy factor based on the half–lives were used to estimate patient release criteria and patient radiation safety instruction thresholds using eqn (4). Results are contained in Tables 5 and 6. Data are offered in both U.S. and system international (SI) units to maintain consistency with Tables U.1 and U.2 of Appendix U of NUREG 1556 Vol. 9 Rev 2.
Exposure rate constants are available from a variety of sources and may differ substantially, as presented in Fig. 2. Use of exposure rate constants in estimating radiation dose to members of the public for patient release requires a consistent methodology. Derivation of the exposure rate constant is based on the μen in air for the energy of interest, the abundance of that energy, and the density of air. This work reviews the basis for calculating these exposure rate constants similar to the methodology offered in U.S. regulatory guidance.
Table 1 presents the μen in air based on a direct visual interpretation of the log log graph of μen found on page 135 of the Radiological Health Handbook (USDHEW 1970). Often, the μen in air is listed with generous increments in energy. The plotted curve indicates a substantial slope for lower energy interactions that could lead to variations when calculating exposure rate constants. The absorption curve was re-created in Microsoft® Office Excel® 2007 and is presented in Fig. 1.
Table 2 compares this estimated μen in air to the ICRU (1970), the RHH (1998), and NBS Handbook (1964) and provides a ratio as a matter of comparison. These current estimated data agree within the largest deviation of 0.89 or a difference of approximately 11% when compared to published resources. In addition, this study offers a tabulated interpretation of μen in increments of 1 keV from 10 keV-40 keV, 2 keV increments from 40 keV-100 keV, 10 keV increments from 100 keV-1 MeV, and 0.1 MeV increments from 1 MeV-3 MeV.
This study reviews the nine radionuclides listed in NUREG 1492 and presents two exposure rate constants from different sources of data (NUREG 1492 and Smith and Stabin). Exposure rate constants were calculated using three methods: 1) using the decay schemes from NUREG 1492, μen from Table 1, and the 11.3 keV photon threshold; 2) using the decay schemes in ICRP 107, μen from Table 1, and the 11.3 keV photon threshold; and 3) using the decay schemes in ICRP 107, μen from Table 1, and energy and abundance thresholds of 15 keV and 0.0001, respectively. It is noted that regulatory guidance exposure rate constants for 103Pd, 125I, and 192Ir presented in NUREG 1492 take into consideration capsule attenuation for sources used routinely in interstitial implants. The regulatory guidance duly notes these sources as “implant’ within their tables. The current study does not include a model for encapsulated sources.
Fig. 2 shows good agreement when comparing the various iterations of exposure rate constants with several radionuclides: 111Ag, 67Ga, 153Sm, and 99mTc. This suggests concurrence between the decay energies and abundances listed in NUREG 1492 and ICRP 107 and good agreement among the μen used by NUREG 1492, Smith and Stabin, and Table 1.
Palladium-103 (103Pd) and 169Yb data appear to diverge when the decay schemes of ICRP 107 are used. For 103Pd, seven photon energies are used in the calculation of the exposure rate constant in NUREG 1492. While 89 photons are listed in ICRP 107 (including decay of the progeny nuclide, Rhenium-103m), 29 are used to calculate the gamma constant with the 11.3 keV threshold, and only 16 are used after the energy threshold is raised to 15 keV and photons with an abundance less than 0.0001 are omitted. Although the number of photons using ICRP 107 data is greater than those in NUREG 1492, the abundance of some photons presented in NUREG 1492 is greater than those contained in ICRP 107 and therefore lead to the higher estimated exposure rate constant. The 103Pd exposure rate constant presented in NUREG 1492 takes into account the attenuation of photons within the implant capsule; however, the exposure rate constant continues to be greater than the others estimated. Based on estimation of the exposure rate constant using the NUREG 1492 decay energies and abundances, the current μen and the 11.3 keV energy threshold, this comparison confirms the NUREG 1492 value to be higher than ICRP decay schemes. This method does not consider attenuation or buildup in a source capsule.
Ytterbium-169 (169Yb) exposure rate constants from NUREG 1492 are less than those calculated using ICRP decay schemes. NUREG 1492 lists 15 photon energies for 169Yb. ICRP 107 lists 126 photon energies, of which 84 meet the photon threshold of 11.3 keV and 35 meet the 15 keV and 0.0001 abundance thresholds. The sum of the contributions of the additional photons listed in ICRP 107 contributes to the higher exposure rate constant.
Tellurium-201 (201Tl) exposure rate constant comparisons are similar, except when the exposure rate constant is derived using ICRP 107 decay schemes combined with the photon energy threshold of 11.3 keV used in NUREG 1492. NUREG 1492 lists seven photon decay energies. The greatest contributor to energy in air is a 71 keV photon with an abundance of 0.47. ICRP 107 lists 70 photons for 201Tl, 55 of which exceed the 11.3 keV threshold, and 16 photons with the 15 keV energy and 0.0001 abundance thresholds. The 39 photons from 11.3–15 keV contribute significantly to elevate the exposure rate constant using ICRP 107 decay schemes. These photons were not listed in NUREG 1492. These energies are then omitted with the higher energy threshold of 15 keV, bringing the exposure rate constant in line with the other methods.
Smith and Stabin (2012) exposure rate constants for 125I and 117Sn are greater than estimated in NUREG 1492 and greater than the results of the methods described here. Comparing the method using ICRP 107 decay schemes, μen from Table 1, and the same energy and abundance thresholds of 15 keV and 0.0001, respectively, the current results are approximately 11.5% lower for 125I and 117Sn. The only variable that cannot be compared is the μen in air. Smith and Stabin (2012) use a log-log interpolation of the data as compared to the authors’ visual interpretation of the linear energy absorption curve as listed in Table 1.
Estimated exposure rate constants from NUREG 1492, NUREG 1556 Vol. 9. Rev. 2 Appendix U, Smith and Stabin (2012), and the current work using ICRP 107 decay schemes, μen in air from Table 1 and the energy and abundance thresholds of 15 keV and 0.0001, respectively, are presented in Table 3. Notable variations in exposure rate constants were discussed in the preceding paragraphs. NUREG 1556 Vol. 9. Rev. 2 Appendix U data are similar to NUREG 1492 with the exception of 103Pd. NUREG 1556 Vol. 9. Rev. 2 Appendix U uses an “apparent” activity for 103Pd, and NUREG 1492 takes into consideration attenuation by the source capsule. Although one might assume that these values should be similar, they are not, and the NUREG 1492 value compares favorably to Smith and Stabin and this work—both of which do not consider attenuation by any source capsule. NUREG 1556 Vol 9. Rev. 2 Appendix U takes into consideration attenuation from the source capsule with 125I as well, impacting the exposure rate constant to a lesser extent.
The exposure rate constant data appear to agree with some explainable discrepancies. Based on these comparisons and the fact that the authors’ novel PET radionuclides are not contained in source capsules, the current model is verified and considered appropriate for generation of exposure rate constants for novel PET radionuclides and further estimating release criteria and thresholds for patient instruction.
This method (ICRP 107 decay schemes, μen from Table 1, and the energy and abundance thresholds of 15 keV and 0.0001, respectively) is used to generate the exposure rate constants for novel PET radionuclides as listed by Holland et al. (2010). These exposure rate constants compare favorably to Smith and Stabin (2012) with a ratio of 1.02 or a variation of 2% or less (see Table 4).
The current exposure rate constants for novel PET radionuclides are entered into the patient release model (eqn 4) to develop activities and dose rate thresholds for patient release and instruction. Results of activity and dose rate are presented in Table 5 for patient release and Table 6 for patient instruction in a similar format (U.S. and SI units) as found in NUREG 1556 Vol. 9 Rev. 2 Appendix U. As in the NUREG, it is assumed that 1 R is equal to 10 mSv (1 rem).
The ICRP states that for most diagnostic nuclear medicine procedures, precautions for the public are rarely required (ICRP 2004). They also recognized that the literature has indicated that models used for patient release are overly conservative. However, patient release is based on dose to a member of the public, and the ICRP further states that young children, infants, and visitors who are not engaged in direct care or comforting should be treated as members of the public and subject to the 1 mSv per year dose limit. The international radiation protection community publishes similar public dose limits as the NRC. However, the European Commission and IAEA specifically exempt caregivers from the public dose limits, whereas the NRC regulation imposes an upper limit of 5 mSv. Also, the EURATOM and IAEA recognize the potential for additional manmade exposures to third party populations and offers a dose constraint of 0.3 mSv per episode for third party members of the public. The 0.3-mSv dose constraint is not used as a criterion for threshold for patient instruction, but it is achieved easily by modifying the dose based threshold of eqn (4).
NUREG 1556 Vol. 9. Rev. 2. Appendix U offers a conservative mechanism for demonstrating compliance with the concept of estimating dose to a member of the public. When considering novel PET radionuclides in this model, several radionuclides demonstrate low activities and dose rates for which patient instruction could be required, particularly for licensees who commit to the model procedures in this guidance.
It is recognized that the model routinely used for patient release employs conservative parameters. For example, the model employs the point source exposure rate constant negating any absorption in tissue, whereas application of correction factors for geometry provide a better estimation of exposure rate (Willegaignon et al. 2006, 2007). Similarly, the relationship from roentgens to radiation absorbed dose, to roentgen-equivalent man is assumed to be equal. The distance from the patient to the member of the public and the occupancy factor are constant. Whichever assumptions are chosen, it is recommended that the licensees be consistent in their methodology.
This work reviews the model for patient radiation safety instruction and release presented in NRC guidance and applies the model to several novel PET radionuclides. Even though these radionuclides are used typically in the diagnostic setting, some exhibit low thresholds for radiation safety instruction when considering administered activity or dose rate at 1 m from the patient. Zirconium-89 (89Zr) and 124I are examples of novel PET radionuclides where the activity and dose rate thresholds are fairly low when considering patient instruction. A patient dose rate of 0.04 mSv h−1 at 1 m or an activity remaining in the patient of more than 0.2 GBq with regard to the administration of 89Zr could require a record of radiation safety instruction. Similarly for 124I, a dose rate of 0.03 mSv h−1 at 1 m or an activity remaining in the patient of more than 0.16 GBq may require a record of radiation safety instruction.
It should be recognized that some quantities of novel PET radionuclides in use today reach the threshold for patient safety instruction using conservative model procedures for patient release.
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